Applications that make use of light emission include an optical space transmission system and a rangefinding system.
One known rangefinding system, which measures a distance up to an object in a contactless manner, for example, employs a time-of-flight (TOF) process. According to the TOF process, light is emitted toward the object, and a period of time, which is consumed after the light has been emitted toward the object and until the light bounces off the object and returns, is measured, so that the distance up to the object can be measured based on the period of time and the velocity of light (see Japanese Laid-Open Patent Publication No. 2001-281336, Japanese Laid-Open Patent Publication No. 08-313215, and Documents 1 and 2 discussed below).
Document 1 contains a detailed explanation concerning the timing to emit pulsed lights and the operational timing of two light-detecting devices in a rangefinding system. More specifically, a pulsed light is emitted and not emitted repeatedly for identical periods (by a light-emitting device which is energized at a duty ratio of 50%), and the light-detecting device transfers photoelectrons alternately in two directions in synchronism with the pulsed light, which is emitted and not emitted (see FIG. 1 of Document 1). A period of time consumed until the pulsed light bounces off the object and returns is determined based on the difference between two output voltages of the light-detecting devices.
Document 2 discloses a light-modulation-detection image sensor for allocating photoelectrons made up of background and modulation light components, and photoelectrons made up of a modulation light component, respectively, to corresponding floating diffusions. In particular, Document 2 discloses a technology for reducing the effect of the diffusion carrier and the effect of residual photoelectrons.
Some rangefinding systems and optical space transmission systems employ light-emitting diodes as light-emitting devices. Other rangefinding systems and optical space transmission systems employ semiconductor lasers as light-emitting devices for high-speed optical transmission, and to enable highly accurate rangefinding operations.
When a semiconductor laser is used as a point light source, since it is necessary to take into account problems related to heat generation and power consumption thereof, the output power of the semiconductor laser needs to be reduced if the object is irradiated with continuous light from the point light source over a given period of time.
Reflected light from the object includes not only a signal light component based on the reflected laser beam, but also noise due to sunlight, and shot noise (a noise component of ambient light) of sunlight. If the output power of the semiconductor laser is reduced, as described above, then the noise component of ambient light becomes greater than the signal light component, resulting in a reduction in the S/N ratio. Inasmuch as the object is irradiated with continuous light over the given period (e.g., during one frame period), a time interval corresponding to the above given period needs to be provided between acquisition of a luminance value when the object is irradiated and acquisition of a luminance value when the object is not irradiated, thus making it impossible to achieve synchronism between acquisition of the luminance value when the object is irradiated and acquisition of the luminance value when the object is not irradiated. Therefore, even if a semiconductor laser is used simply as a light-emitting device, it is difficult for the semiconductor laser to be used in various applications that make use of light emission.
Document 1:
Ryohei Miyagawa and Takeo Kanade, “CCD-Based Range-Finding Sensor”, IEEE Transactions on Electron Devices, Vol. 44, No. 10, October 1997, pp. 1648 through 1652.
Document 2:
Koji Yamamoto, Yu Oya, Keiichiro Kagawa, Jun Ohta, Masahiro Nunoshita, and Kunihiro Wanatabe, “An Image Sensor with the Function for Detecting a Modulated Light Signal: Improvement of Image Characteristics Captured by a Modulated Light”, Technical Report from the Institute of Image Information and Television Engineers, Vol. 27, No. 25, Mar. 28, 2003, pp. 9 through 12.